Temperature sensors are utilized in a variety of applications. For example, temperature sensors that are used in conjunction with ovens typically comprise a metallic tube in which a temperature sensitive element is disposed inside one end with conductive wires extending within the tube from the temperature sensitive element to an opening at the other end of the tube. The metallic tube is inserted through a wall of the oven to permit the temperature sensitive element to be placed in thermal communication with the internal cavity of the oven. The temperature sensitive element is typically a resistive temperature detector, or RTD. The temperature sensor can be also based on a thermistor or thermocouple configuration, a metal oxide semiconductor, or any other type of temperature sensing element.
One area where temperature sensors find particular usefulness is in the area of exhaust gas environments. Various applications require measurement of temperature of gas or mixture of gases at elevated temperatures. One such application involves automotive or combustion applications in which a need exists for measuring the exhaust gas temperature for emission control using Selective catalytic reduction (SCR) and Exhaust Gas Recirculation (EGR) based emission after treatment systems. The sensor should function in a harsh and corrosive automotive exhaust gas environment containing, for example, soot particles, SOx, moisture, diesel, NH3, NOx, HC, CO, CO2 etc.
Exhaust gas temperature (EGT) can be utilized to measure the performance, for example, of an automotive engine. The exhaust gas temperature also provides an indication of the rate of deterioration of automotive engine components. Thus, since the exhaust gas temperature is an indicator of engine status, it may be used to measure and control operational and functional characteristics of the engine.
Accurate measurement of the exhaust gas temperature level is important. To accurately measure exhaust gas temperatures, it is necessary to minimize degradation of the EGT measurement system. Thus it is desirable that the EGT measurement system compensate for engine to engine variations and combustor exit temperature profiles. In addition, the measurement system should compensate for shifts in engine profiles that may occur with progressive deterioration of the engine components.
The penetration of a particular sensor can be determined by the temperature profile of the exhaust gases. The exhaust gas temperature profile is determined by the number, type and arrangement of the combustion nozzles in the combustor. The exhaust gas temperature profile for a particular engine may be determined by using a large number of thermocouple elements arranged in a number of sensors around the exhaust passage and at various penetration depths. Once the exhaust gas temperature profile is defined for a particular type of engine, it may be used to calculate the number and arrangement of EGT sensors necessary to monitor the exhaust gas temperature during normal engine operation.
As indicated above, a variety of temperature sensing elements can be utilized in the context of an exhaust gas temperature sensor. Resistance Temperature detectors (RTD) elements can be used in temperature measuring equipment. The RTD Element has a ceramic substrate with a platinum or nickel or similar metal thin/thick film resistor with an over coating of a protective layer like glass or ceramic or any other material glazing, which is thermally a good conductor. Wire wound RTD elements are also available. Materials such as, for example, platinum or nickel have a positive co-efficient of temperature and the resistance increases linearly with increase in temperature.
Thermistors are also utilized in temperature measuring equipment. Thermistors are essentially semiconductor devices, which behave as thermal resistors having high negative or positive temperature co-efficient of resistance. Thermistors are made of sintered metal oxide ceramics like oxides of iron, magnesium, nickel, cobalt and copper in the form of beads or discs or rods. The variation in temperature is non linear, resistance decreases with increase in temperature in case of negative temperature co-efficient (NTC) of resistance thermistor and resistance increases with increase in temperature in case of positive temperature co-efficient (PTC) of resistance thermistor.
Thermocouples are the most commonly utilized temperature sensing devices and operate based on the principle of the so-called See-Beck effect, i.e., when two dissimilar metal or ceramic or metal oxide semiconductor junctions are maintained at different temperature an EMF is induced at the junction, which is proportional to temperature difference. Generally Platinum with copper, Constantan, Nickel, Rhodium, Iron, Gold, ZrO2, Al2O3, CeO2 and so forth can be utilized. The sensing element can be suitably packaged and placed in a gas flow path and the temperature is measured by using a suitable electronic circuit by transduction of resistance or voltage.
Temperature sensors can be configured to include a housing formed from a material, such as, for example, stainless steel, inconel, brass, and so forth. A connecting cable may typically connect to sensor at an interface junction. The junction or interface of the cable and metal tube is generally crimped to hold the cable mechanically. In harsh environments, however, such as automotive applications, there exists a high demand for water and dust proof sealing and often the crimping of such sensors fails to withstand the ingress of water and moisture and corrosive gases and liquids, while being susceptible to leakage. There thus exists a continuing need for temperature sensors that are water proof and leak proof, while also suitable for harsh and long exposure to corrosive environments, such as, for example, an automobile exhaust gas environment.